The present invention relates to a method for forming an optical thin film used as an antireflection film of an optical element used for high-energy lasers, a thin film for enhancing the laser resistance of polarizers and reflectors used for high-energy lasers, and an antireflection film formed on an eye-protection filter for lessening screen glare of a display.
Methods for forming the optical thin film are roughly classified into vapor deposition and chemical processes.
Antireflection films formed by vapor deposition include monolayer films and multilayered films. The monolayer film is formed by vapor-depositing, on a substrate, a material having a refractive index lower than that of the substrate at a thickness of one fourth a wavelength. In the multilayered film, at least two layers are vapor-deposited using materials having a high refractive index and a low refractive index.
The inventors of the present invention have proposed a method using both the vapor deposition and a chemical process. In this method, a water-soluble material and a water-insoluble material are vapor-deposited at one time onto a substrate to form a mixed film, and then the water-soluble material is removed from the mixed film to form a porous thin film comprising the water-insoluble material.
The method for forming the porous thin film has been disclosed in Japanese Patent Application Publication No. 5-52923.
Milam and others of Lawrence Livermore National Laboratory in the U.S. have developed a method for forming an antireflection film using a chemical process and in which a porous silica thin film having a reflectance of 0.1% to 0.3% is formed on the surface of a quartz glass substrate by a sol-gel process. This method has been disclosed in D. Milam et al., CLEO""84 Technical Digest, THB 2 (1984).
Thomas of the same laboratory has formed porous MgF2 and CaF2 fluoride thin films on quartz glass and CaF2 crystal substrates by a sol-gel process. This method for forming the porous MgF2 and CaF2 films has been disclosed in Ian M. Thomas, Appl. Opt., Vol. 27, No. 16, pp. 3356-3358 (1988).
However, these methods described above have the following problems.
Optical films formed by the known vapor deposition are less resistant to laser treatment. In addition, it is very difficult to form an antireflection film for ArF and F2 lasers having emission wavelengths of 193 nm and 157 nm, respectively, which are deep ultraviolet light used for lithography, because, in order to form a broadband antireflection film, at least three layers must be vapor-deposited.
Furthermore, once a film is damaged, the substrate having the damaged film needs to be restored through two processes, rough polishing and ultra-fine polishing.
This is because the film formed by the known vapor deposition locally has, between the surface of the substrate and the deposited thin film, an absorption layer which cannot be removed by ultrasonic cleaning or laser cleaning, that is, laser light exposure, and because the remaining absorption layer is turned into plasma by a high-energy laser light to destruct the deposited film.
A broadband antireflection film is formed by vapor-depositing two materials having a high refractive index and a low refractive index in 3 to 7 layers, or by depositing two materials in 100 to 300 layers each having a thickness of one hundredth to one three hundredth that of a monolayer film. However, these methods increase manufacturing costs in comparison with a method for forming a monolayer film.
Also, when a damaged film is removed from a substrate to reuse the substrate, it takes at least about 5 hours to restore the substrate because of the two polishing processes.
The method proposed by the inventors, using the mixed film and the chemical process to form a porous thin film whose refractive index has a gradient is applicable to oxide, but the method is difficult to apply to fluoride. In addition, two materials must be vapor-deposited while the mixing ratio of the materials is varied in the process of forming the mixed film. However, this makes it difficult to control the deposition rate and thus makes it difficult to steadily obtain a desired reflectance.
The porous MgF2 and CaF2 films formed by the sol-gel processes proposed by Milam or Thomas have a reflectance of 0.5% or less at a predetermined wavelength and a laser resistance two or more times than that of a thin film formed by vapor deposition. However, the surfaces thereof undesirably exhibit a low mechanical strength.
This is because the porous thin films are formed of colloidal particles adhered onto the surface of a substrate by Van der Waals force and therefore easily peel if an external mechanical force is applied to the thin film.
Accordingly, the present invention is intended to solve these problems and the object of the present invention is to provide a method for forming an optical thin film used as an optical element used for laser systems including high-energy lasers and an optical element used for optical apparatuses. The optical film has a high laser resistance and a reasonable hardness at the surface thereof, and can be used with a desired refractive index gradient in a broad wavelength band from deep ultraviolet to infrared. In particular, the method provides a porous fluoride thin film for preventing reflection in a deep ultraviolet region, easily vapor-deposited on a desired substrate with good reproductively. Also, the thin film can be easily removed to reuse the substrate if the film is damaged.
In order to achieve the object, the present invention provides the following:
(1) A method for forming an optical thin film comprises the steps of: vapor-depositing a water-insoluble material for preventing reflection onto an optical element substrate; vapor-depositing a water-soluble material having a higher particle energy onto the surface of the water-insoluble material; allowing the water-soluble material to permeate deep into the water-insoluble material to form a mixed film on the surface of the substrate; and subsequently dissolving and removing the water-soluble material to form a porous thin film comprising the water-insoluble material.
(2) In the method for forming an optical thin film described in (1), the optical element substrate comprises one material selected from the group consisting of: optical glasses including quartz glass, borosilicate crown glass, and phosphate glass; crystals including fluorite, rock crystal, and sapphire; laser crystals including YAG and Al2O3; ceramics; semiconductors; plastics; and metals.
(3) In the method for forming an optical thin film described in (1), the water-insoluble material comprises one compound selected from the group consisting of: oxides including silica; and fluorides including magnesium fluoride.
(4) In the method for forming an optical thin film described in (3), the oxides and the fluorides consist of SiO2, Al2O3, CeO2, HfO2, Ta2O5, ThO2, TiO2, ZrO2, Sc2O3, MgF2, AlF3, CaF2, LiF, LaF3, PbF2, and NdF3.
(5) In the method for forming an optical thin film described in (1), the water-soluble material comprises one compound selected from the group consisting of fluorides, oxides, chlorides, and phosphates.
(6) In the method for forming an optical thin film described in (5), the fluorides, the oxides, the chlorides, and the phosphates consist of NaF, Na3AlF6, LiF, B2O3, MgCl2, NaCl, NiCl2, LaCl3, LiCl, and NaPO3.
(7) The method for forming an optical thin film described in (1) further comprises the step of forming an overcoat film on the surface of the porous thin film.
(8) In the method for forming an optical thin film described in (7), the overcoat film comprises a fluoride or an oxide and is 50 to 500 xc3x85 in thickness.